1. Field of the Invention
The present invention relates to an audio amplifier, and more particularly, to a quaternary/ternary modulation selecting circuit of an audio amplifier, and an associated method.
2. Description of the Prior Art
In audio applications, an analog amplifier is one of the most important parts in an audio system. Efficiency is a critical issue; a class D power amplifier is the most popular kind used in audio systems due to its higher efficiency compared to other kinds of amplifier. Because the output waveform of the class D power amplifier is a modulation signal between two voltage levels (i.e. supply voltage and ground) rather than a common linear waveform, ideally no current will pass through when the transistors of the output stage are non-conductive. Currently, a common modulation method applied in the class D power amplifier is Pulse Width Modulation (PWM), wherein quaternary modulation has the advantages of better total harmonic distortion (THD) and lower noise; ternary modulation, however, has the advantages of better efficiency and better Electro Magnetic Interference (EMI) performance. Therefore, a novel architecture of the class D amplifier which can combine the above advantages and can be utilized the ternary modulation and the quaternary modulation according to different power conditions is required.